Characterization of active sites in metalloenzymes such as RR (ribonucleotide reductase) and MMO (methane monooxygenase) by electron paramagnetic resonance (EPR) is structurally informative if complete EPR signals can be obtained. But partial EPR signals are found for many high-spin metal ions and metal ion clusters in metalloproteins and model systems. In this case, the method of Hendrich and Debrunner, involving measurement of the partial EPR signals in both normal mode and parallel-mode experiments (oscillating magnetic field) gives the information necessary for quantitation of the signals from the metalloprotein sites and ultimately for testing structural models. This technique is applied to several protein and model system sites. The striking success of this method has stimulated a major instrument supplier, Bruker, to produce parallel-mode accessories for their EPR systems. This is a multi-year ongoing project.